There are numerous techniques in common usage for forming the electrical interconnections between the bond pads of a semiconductor die and a lead frame or other substrate. One of the oldest and still most common techniques is to form a wire bond between a die and its substrate using a wire bonding machine. The wire bonding machine forms a wire through the air between the die bond pad and the appropriate electrical connection point on the substrate (e.g., a lead finger of a lead frame).
A typical wire bond is made by “ball bonding” one end of a gold wire to a bond pad on the die and “stitch bonding” the other end of the wire to the lead frame. More particularly and referring specifically to FIG. 1A, using a ball bonding machine 10 (of which the Figures only show the capillary and the wire protruding from the capillary bore hole), a wire 12 (commonly, but not necessarily gold) is passed through the bore of a capillary 14 of the wire bonding machine 10. The capillary 14 is positioned above the bond pad 16 of the die 18 with the clamps 19 closed to prevent the wire from moving relative to the capillary. The end of the wire protruding from the capillary is heated by means of an electric spark, termed “electric flame-off”, to melt the end of the wire. The end of the wire inherently forms into a ball 20 when melted. FIG. 1A shows the condition of the ball bonding apparatus at this point of the process.
With the clamps still closed so that the wire cannot pay out of the capillary or slide back within the capillary, the capillary 14 is then moved downwardly so that the ball 20 comes into contact with the bond pad 16 on the die and squeezes the ball between the tip of the capillary and the bond pad. Heat and/or ultrasonic energy are applied to the die 18 to facilitate the ball 20 becoming bonded to the bond pad 16. This bond is commonly termed a ball bond or first bond. Then the clamps 19 are opened. FIG. 1B shows the condition of the ball bonding apparatus at this point of the process. The capillary 14 is then raised above the first bond as shown in FIG. 1C.
Next, the capillary 14 is moved to a second bond site 22, this one on the substrate 24, with the wire 12 (which is still connected to the ball bond) trailing out of the capillary. The motion of the capillary between the first bond site and the second bond site may include any particular motion components desired to cause the wire to form a loop of appropriate shape between the first and second bond sites. At the second bond site 24, the capillary contacts the substrate 24 to pinch the wire 12 between the tip of the capillary 14 and the substrate 24, as shown in FIG. 1D. Again, heat and/or ultrasonic energy may be applied to facilitate bonding the compressed portion of the wire 12 to the substrate 24. This bond is commonly termed a stitch bond or second bond. At this point, the wire has been pinched, but has not been fully cut. The capillary 14 then rises with the wire 12 still attached to the stitch bond and with the clamps still open such that additional wire pays out of the capillary. The clamps 19 are then closed and the capillary 14 rises further to snap the wire at the weakened point at the stitch bond as shown in FIG. 1E. The completed interconnection, indicated by reference numeral 23 in FIG. 1E, is termed a “wire loop”.
At this time, the capillary is moved to a position above the next bond pad on the die and the wire that is sticking out of the tip of the capillary after the completion of the last step is melted by electric flame-off to form the next ball for forming the next wire bond.
The ball bond is typically made on a bond pad on a semiconductor die and the stitch bond is made on the substrate, such as illustrated in FIGS. 1A-1E. Normally, it is inadvisable to make the stitch bond on the bond pad of the die because, in order to make a stitch bond, the capillary tip essentially must come in contact with the surface in and surrounding the second bond site. If the capillary contacts the top of a die, it likely will damage circuitry on the die. On the other hand, the capillary typically can come in contact with the substrate without a problem since the capillary typically will not harm the substrate.
Nevertheless, it is sometimes desirable to make a stitch bond on a die. One such situation is when, instead of connecting a bond pad of a die to a lead element, one needs to interconnect a bond pad on one die to a bond pad on another die (hereinafter die-to-die bonding). In this situation, both ends of the wire loop are on bond pads of dies. If one wishes to use a ball bonding machine to make a die-to-die wire loop, one of the dies must receive a stitch bond rather than a ball bond.
Another situation where it may be desirable to stitch bond to a bond pad of a die is when a very low-profile (i.e., thin) semiconductor die package is desired. This is because the highest point of a wire loop is close to the ball bond, as can be seen from FIG. 1E. The bond pad 16 on the top surface of the die 18 is at a higher elevation than the substrate 24 because the die 18 sits on top of the substrate. Accordingly, in low profile packaging situations, in order to minimize the height of the entire package, it is sometime desirable to make the ball bond on the lead frame and the stitch bond on the die (hereinafter reverse bonding) so that the highest point of the wire near the ball bond is at a lower elevation than in standard forward wire bonding techniques. Accordingly, the overall height of the package can be lessened. However, it usually would not be possible to form a stitch bond on a bond pad on a die for the reasons discussed above, namely, the capillary might damage surrounding circuitry on the die. when it contacts the die to form the stitch bond. Even further, as shown in FIG. 2, which is an enlarged profile of a typical wire loop 23, the wire tends to sag to its lowest point 26 close to the stitch bond site 22. If the stitch bond site is higher than the ball bond site, such as would be the case in reverse bonding, the wire might contact the edge or the top surface of the die or even other wires leading to electrical shorts or breakage of the wire.
Accordingly, techniques have been developed to ameliorate this problem. One such technique is to form a bump on top of the bond pad on the die and then form the stitch bond on top of that bump so that the capillary tip contacts the top of the bump rather than the die. The bump may be formed, for instance, using the same wire bonding machine that forms the loop. Specifically, the wire bonding machine can be used to place a ball bond on the die pad in the normal fashion, but, instead of then paying out the wire to form a wire loop and stitch bonding the wire loop at a second location, the wire is severed right at the top of the ball leaving just the ball bond on the bond pad. Specifically, the wire can be severed by (1) raising the capillary with the clamps open after forming the ball bond, (2) closing the clamps, and (3) raising the capillary further to snap the wire off at the weak point at the top of the ball. A reverse boding process can then be performed with the first/ball bond of the loop formed on the substrate and the second/stitch bond of the loop formed on top of the bump on the bond pad.
As a result of the ever-present drive toward making electronic components smaller and smaller, the diameters of the wires used in wire bonding, the size of the ball bonds and the pitch (spacing) of the bond pads on dies are all decreasing. Further, packages are becoming thinner and thinner. Accordingly, present techniques for reverse bonding are becoming less and less effective. Particularly, it is becoming increasingly difficult in reverse bonding to achieve sufficient height clearance of the wire loops to reasonably assure that the wires will not contact the top or edge of the die or adjacent wires. Also, with smaller and smaller bumps, it is becoming increasingly difficult to form reliable stitch bonds on top of the bumps. One technique that has been employed to help improve the reliability of stitch bond formation on the tops of bumps is to smooth the top of the bump during its formation. For instance, one particular technique is to (1) form a ball bond on the pad in the normal fashion by melting the wire, moving the capillary down to press the melted ball onto the bond pad and applying heat and/or ultrasonic energy, (2) raising the capillary slightly (on the order of a fraction of the thickness of the wire) so that the capillary tip is still within the ball, (3) moving the capillary laterally or diagonally downward while applying ultrasonic energy such that the capillary tip partially cuts through the gold near the top of the ball and smooths the top of the ball, (4) raising the capillary with the clamps opened so that the wire (which is still attached to the top of the ball) pays out of the capillary, (5) closing the clamps, and (6) raising the capillary further to pull the wire causing the wire to snap off at the weak point created at the top of the ball. FIG. 3 shows a bump 30 formed by this process. The bump essentially comprises a ball bond 31 with a sloped, smoothed surface 32 on top and a nub 33 where the wire was severed. Bumps formed by this process tend to have more uniform height and a large, smooth surface more amenable to reliable stitch bonding thereon. However, with very fine wires and/or fine pitch applications, it is still difficult with this technique to assure adequate clearance between the wire and the die.
A diagonal motion to smooth the top of the ball often is preferable to a strictly lateral motion. Particularly, while the purely lateral smoothing motion worked well for many applications, it was found that, as ball sizes and pitches continued to decrease in size, the force exerted by the capillary tip on the bump during the purely lateral smoothing motion could tear the bump off of the pad. In either embodiment, since a portion of the top of the bump was being scraped off, this technique often suffered even greater problems of inadequate clearance.
Formation of bumps for electrical interconnection of semiconductor devices to substrates is used in another well known technique termed flip chip bonding. In flip chip techniques, the semiconductor die is “flipped” such that its “top” surface (i.e., the surface bearing the bond pads and circuitry) faces downwardly. Bumps are formed on the substrate in positions so as to mate with bond pads on the die. The upside down die is pressed down onto the substrate so that the die is suspended upside-down on the substrate with the bond pads in electrical contact with the stud bumps on the substrate such that the remaining circuitry on the die does not contact the substrate. Various techniques for forming the stud bumps on the substrate are well known, including forming such bumps with a wire bonding machine essentially in the way described above for forming bumps for reverse wire bonding. In flip chip interconnect techniques, there is a minimum bump height necessary to assure adequate clearance between the surface of the substrate and the surface of the die to avoid electrical shorting. As wire diameters and pitches of the bond pads continue to decrease, it is becoming more and more difficult to form bumps of sufficient height and uniformity with a wire bonding machine.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for forming bumps for semiconductor electrical interconnection applications.
It is another object of the present invention to provide an improved bump for semiconductor electrical interconnection applications.
It is a further object of the present invention to provide an improved method and apparatus for forming bumps for semiconductor electrical interconnection applications using a wire bonding machine.
It is a yet a further object of the present invention to provide an improved method and apparatus for performing reverse bonding using a wire bonding machine.